Global navigational satellite systems (GNSS) include the global positioning system (GPS) and the Russian global orbiting navigational satellite system (GLONASS). GNSS-based navigational systems are used for navigation and positioning applications. In the GPS navigational system, GPS receivers receive satellite positioning signals transmitted from a set of up to 32 satellites deployed in 12-hour orbits around the earth and dispersed in six orbital planes at an altitude of 10,900 nautical miles. Each GPS satellite continuously transmits two spread spectrum, L-band signals: an L1 signal having a frequency f1 of 1575.42 MHz, and an L2 signal having a frequency f2 of 1227.6 MHz. The L1 signal from each satellite is modulated by two pseudo-random codes, the coarse acquisition (C/A) code and the P-code. The P-code is usually encrypted, with the encrypted version of the P-code referred to as the Y-code. The L2 signal from each satellite is modulated by the Y-code. The C/A code is available for non-military uses, while the P-code (Y-code) is reserved for military uses.
A GPS receiver usually determines its position using the travel time of a coded radio GPS signal received from a particular satellite. The receiver can generate or maintain a set of codes identical to those codes (e.g., the Y-code or the C/A-code) transmitted by the satellite. The receiver can compute the travel time of the received coded radio GPS signal by determining a time shift between its own codes and the codes transmitted by the satellite. The receiver can multiply the computed travel time by the speed of light to determine the distance between the transmitting satellite and the receiver. The receiver can receive GPS signals from four or more satellites and determine how far it is from each of the satellites. Using its distance from the various satellites, the receiver can accurately determine its position in three dimensions (e.g., longitude, latitude, and altitude). A conventional GPS receiver typically utilizes the fourth satellite to accommodate a timing offset between the clocks in the receiver and the clocks in the satellites. The GPS signals also include a 50 bit per second data stream or data message which is superimposed on the C/A and Y-codes. Once the receiver has matched its code to the code in the GPS signal from a particular satellite, the receiver can decipher the data message. The data message can include navigational data related to the position of the satellite, including geometric dilution of precision (GDOP) parameters. Additionally, the data message can include accurate time data, ephemeris data, and data related to the health status of the satellite. The GPS satellites utilize code division multiple access techniques so satellite signals do not interfere with each other. GLONASS navigational systems operate similarly to GPS navigational systems and utilize frequency division multiple access (FDMA) techniques so satellite signals do not interfere with each other.
GNSS navigational systems have tremendous benefits over other positioning and navigational systems because these systems do not rely upon visual, magnetic or other points of reference. However, conventional GNSS navigational systems are susceptible to jamming or interference by other signals. When jammed by higher power signals, the C/A-code encoded signal becomes difficult to acquire. Typically C/A-code acquisition is necessary to acquire the Y-code signal. Therefore, in the presence of jamming or interfering signals, navigation using a GNSS receiver can become unreliable.
In some applications, such as a military battle field, pseudolites can be used to enhance GNSS performance when satellite signals undergo jamming, interference, or other phenomena rendering signal reception unreliable. For example, pseudolites transceivers that are away from a jamming environment can receive GPS signals from satellites, determine their positions based on the GPS signals received from the satellites, and generate and broadcast pseudolite positioning signals that can be received by GNSS receivers. Also, a stationary pseudolite can broadcast pseudolite positioning signals to GNSS receivers within a given distance from the stationary pseudolite. The pseudolite positioning signals can allow GNSS receivers that are unable to track the GNSS satellite signals (e.g., due to jamming or other reasons) to perform relative position navigation. In other words, the GNSS receivers in such a case can determine their positions with respect to the locations of the transmitting pseudolites.